Cardiac arrhythmias, the most common of which is ventricular tachycardia (VT), are a leading cause of death. In a majority of patients, VT originates from a 1 mm to 2 mm lesion located close to the inner surface of the heart chamber. One of the treatments for VT comprises mapping the electrical pathways of the heart to locate the lesion followed by ablation of the active site.
U.S. Pat. No. 5,546,951; U.S. patent application Ser. No. 08/793,371; and PCT application WO 96/05768, which are incorporated herein in their entirety by reference, disclose methods for sensing an electrical property of heart tissue such as local activation time as a function of the precise location within the heart. The data are acquired by advancing into the heart one or more catheters that have electrical and location sensors in their distal tips. The precise three-dimensional location of the catheter tip is as ascertained by the location sensor contained therein. The location sensor operates by generating signals that are responsive to its precise location within an externally generated non-ionizing field such as an electromagnetic field. Simultaneous with the acquisition of location information, electrical information is also acquired by at least one electrode contained at the catheter distal tip. Accurate sensing of location and electrical information by sensors contained in the catheter generally requires a high degree of confidence that a catheter electrode is in contact with the tissue.
Methods of creating a map of the electrical activity of the heart based on these data are disclosed in U.S. patent application Ser. Nos. 09/122,137 and 09/357,559 filed on Jul. 24, 1998 and Jul. 22, 1999, respectively, and in European Patent Application 974,936 which are also incorporated herein in their entirety by reference. In clinical settings, it is not uncommon to accumulate data at 100 or more sites within the heart to generate a detailed, comprehensive map of heart chamber electrical activity. The use of the location sensors as hereinabove described is highly useful in providing a detailed and accurate map of the heart chamber's activity.
Catheters containing position or location sensors may also be used to determine the trajectory of points on the cardiac surface. These trajectories may be used to infer mechanical motion characteristics such as the contractility of the tissue. As disclosed in U.S. Pat. No. 5,738,096 which is incorporated herein in its entirety by reference, maps depicting such motion characteristics, which may be superimposed with maps depicting local electrical information, may be constructed when the trajectory information is sampled at a sufficient number of points in the heart. Accurate maps of such motion characteristics again require confidence that the data are acquired when the catheter tip is in contact with the cardiac tissue.
The detailed maps generated as hereinabove described may serve as the basis for deciding on a therapeutic course of action, for example, tissue ablation, to alter the propagation of the heart's electrical activity and to restore normal heart rhythm. In cardiac ablation, energy, typically in the radiofrequency (RF) range, is supplied at selected points on the intracardiac surface by a catheter having an ablation electrode at its distal tip. Ablation is effected by bringing the distal tip electrode into contact with the locus of aberrant electrical activity and by initiating the delivery of RF energy through the distal tip electrode from an external RF generator in communication with the distal tip electrode. Ablation is most effectively performed when the distal tip electrode is in contact with the cardiac wall. Absence of contact or poor contact of the tip electrode with the heart wall leads to dissipation of the RF energy in the blood, as well as possible fouling of the tip electrode with the concomitant possibility of blood clot formation. Accordingly, it is important that both mapping and ablation be accompanied by methods and systems for detecting and ensuring electrode-tissue contact.
A number of references have reported methods to determine electrode-tissue contact, including U.S. Pat. Nos. 5,935,079; 5,891,095; 5,836,990; 5,836,874; 5,673,704; 5,662,108; 5,469,857; 5,447,529; 5,341,807; 5,078,714; and Canadian Patent Application 2,285,342. A number of these references, e.g., U.S. Pat. Nos. 5,935,079, 5,836,990, 5,447,529, and 6,569,160 determine electrode-tissue contact by measuring the impedance between the tip electrode and a return electrode. As disclosed in the '160 patent, the entire disclosure of which is hereby incorporated by reference, it is generally known that impedance through blood is generally lower that impedance through tissue. Accordingly, tissue contact has been detected by comparing the impedance values across a set of electrodes to pre-measured impedance values when an electrode is known to be in contact with tissue and when it is known to be in contact only with blood.
A disadvantage of typical ablation electrodes is that it is sometimes difficult to accurately predict lesion size because the lesion size can vary depending on the orientation of the ablation electrode. For example, typically a 7 French catheter (having an outer diameter of just over 2 mm) is provided with an ablation tip electrode at its distal end having a length ranging from about 4 mm to about 8 mm. If the ablation electrode is provided in perpendicular relation to the tissue, a relatively small surface area of the electrode is in contact with the tissue. In contrast, a relatively larger surface area would be in contact with the tissue if the ablation electrode were in a generally parallel relationship to the tissue, i.e., if the ablation electrode were positioned on its side. The size of the lesion is often related to the amount surface area in contact with the tissue and there can be significant energy loss through the portion of the electrode not in contact with any tissue, particularly since blood has a higher conductivity than tissue.
Moreover, overheating of the tip electrode can cause complications, including the formation of char and/or thrombus on the electrode surface. The creation of char and thrombus is unsafe, as the char and thrombus can be dislodged from the electrode during the procedure or during removal of the catheter after the procedure.